My friend Applo and I write quite a few collabs together for other rps. Basically we put down what we want and then look over to see for repetitions, grammar, typos, as well as the flow so it reads like a story, sometimes breaking our own paragraphs to add in reactions or dialogue where it seems natural. 

I personally find it better than solo posts at certain times because otherwise it seems repetitive when someone is simply reiterating what another person may have already posted.

^_^' Sorry if it seems like I'm butting in! Just thought I'd add something seeing I only recently started writing collabs this year.